Lasers with low doped gain medium

ABSTRACT

A high power, diode pumped laser has a Nd:YVO 4  gain media. Scaling to higher powers is achieved with the use of a low doped gain media, increasing the length of the gain media as well as increasing the pump volume. Passive cooling is extended to output powers of 10 W or greater .

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to diode pumped Nd:YVO₄ lasers, and moreparticularly to diode pumped Nd:YVO₄ lasers with Nd doping levels ofless than 0.5%.

2. Description of Related Art

The most common gain media used for diode pumped lasers is Nd:YAG andefficient systems can be constructed by end pumping with laser diodesand laser diode arrays. To build an efficient end pumped Nd:YAG laser,the pump light from the diode, which is typically not in a diffractionlimited beam, must be focussed tightly into the gain media. To obtainTEM₀₀ operation, which is desirable for many applications, the pumplight must be focussed to a spot size smaller than the intracavity mode.In addition, since the pump light diverges more quickly than theintracavity mode, it must be absorbed in a short distance before it willdiverge to a size larger than the intracavity mode. Thus tight focussingand short absorption depths were necessary to build efficientTEM₀₀Nd:YAG lasers pumped by diode lasers and diode arrays. Thesetechniques are described in U.S. Pat. Nos. 4,635,056; 4,701,929; and4,756,003.

The pump power available from these diode pump sources has increasedsteadily from 1 W diodes to 20 W diode bars and most recently to 40 Wbars at 809 nm. As the pump power increased, several problems wereencountered scaling the Nd:YAG lasers to higher power. For the YAG hostin particular, increased pump power per unit area leads to increasedbirefringence. The gain media depolarizes the intra cavity beam; thisleads to losses when polarized output is desired. A solution to thebirefringence problem is to substitute Nd:YLF as the gain media. YLF isa birefringent material and naturally produces polarized output, evenunder high thermal loading. YLF, however, suffers from fracture problemsas the pump power and hence the thermal loading is increased. Analternative material which is also naturally polarized and lesssusceptible to fracture is Nd:YVO₄ (Nd:Vanadate or Vanadate).

As the pump power incident on the Vanadate crystal is increased, thermallensing becomes the limiting factor. At high pump powers the lensbecomes very strong with focal lengths as short as 10 cm. Although thisstrong lens can be largely compensated by clever cavity design, theaberrations in the lens eventually degrade the performance of the laser.Thus, in order to take advantage of the new higher power diode bars aspump sources, a solution to the aberrated thermal lens in Vanadate isneeded.

The power of the lens in a diode pumped Vanadate laser is due to twomajor contributions: the lens due to the index change in the bulk andthe lens due to the bulge in the surface of the crystal. One solution toreducing the surface bulge is to optically contact undoped Vanadate onthe end of the gain media. These end caps do not reduce the lens in thebulk however, which is the subject of the following disclosure. Anothertechnique to reduce the surface bulge is to pass the pump light throughthe crystal more than once. For example, a highly reflective coating forthe pump light can be placed on the second surface of the crystal. Thepump light will then pass twice through the crystal causing the pump tobe absorbed more homogeneously throughout the crystal and causing lessheating near the surface. Either of these techniques may be used incombination with the method described below to reduce the thermal lenseven further.

There is a need for a Vanadate laser or laser system with higher powers.There is also a need for a Vanadate laser or laser system with a reducedlens in the bulk of the crystal.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to provide a diode-pumpedNd:YVO₄ laser.

Another object of the invention is to provide a diode-pumped Nd:YVO₄laser that is scalable to high power.

Yet another object of the invention is to provide a high powerdiode-pumped Nd:YVO₄ laser with a TEM₀₀ beam with high efficiency.

A further object of the invention is to provide a compact diode-pumpedNd:YVO₄ laser.

Yet another object of the invention is to provide a diode-pumped Nd:YVO₄laser that is passively cooled.

These and other objects of the invention are achieved in a diode pumpedlaser with a first high reflector mirror and an output coupler thatdefines a resonator cavity. A first Nd:YVO₄ gain medium is positioned inthe resonator cavity. The gain medium has an Nd doping level of lessthan 0.5% and a length of at least 4 mm. A first diode pump sourcesupplies a first pump beam that is incident on a first pump face of thefirst Nd:YVO₄ gain medium.

In another embodiment of the invention, a diode pumped laser has a firsthigh reflector mirror and an output coupler that defines a resonatorcavity. A first Nd:YVO₄ gain medium is positioned in the resonatorcavity with a pump volume of at least 8 mm³. A first diode pump sourcesupplies a first pump beam that end pumps a first pump face of the firstNd:YVO₄ gain medium.

In another embodiment of the invention, a diode pumped laser includes afirst high reflector mirror and an output coupler that define aresonator cavity. A first Nd:YVO₄ gain medium is positioned in theresonator cavity. The Nd:YVO₄ gain medium has a length greater than 8mm. A first diode pump source supplies a first pump beam that isincident on a first pump face of the first Nd:YVO₄ gain medium.

In another embodiment of the invention, a diode pumped laser includes afirst high reflector mirror and an output coupler that define aresonator cavity. A first Nd:YVO₄ gain medium is positioned in theresonator cavity. The first Nd:YVO₄ gain medium has a doping level and apump volume that permit the first Nd:YVO₄ gain medium to be passivelycooled. A first diode pump source supplies a first pump beam that isincident on a first pump face of the first Nd:YVO₄ gain medium. Thelaser produces an output beam with a power of at least 5 watts at 532nm.

In another embodiment of the invention, a diode pumped laser includes afirst high reflector mirror and an output coupler that define aresonator cavity. A first Nd:YVO₄ gain medium is positioned in theresonator cavity. The first Nd:YVO₄ gain medium has a doping level and apump volume selected to permit the first Nd:YVO₄ gain medium to bepassively cooled. A first diode pump source supplies a first pump beamthat is incident on a first pump face of the first Nd:YVO₄ gain medium.The laser produces an output beam with a power of at least 10 watts at1064 nm.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a laser of the present invention with asingle Nd:YVO₄ gain medium pumped by a first diode pump source.

FIG. 2 is a schematic diagram of a laser of the present invention with asingle Nd:YVO₄ gain medium pumped by a two diode pump sources.

FIG. 3 is a schematic diagram of a laser of the present invention with afirst Nd:YVO₄ gain medium pumped by first and second diode pump sources,and a second Nd:YVO₄ gain medium pumped by third and fourth diode pumpsources.

FIG. 4 is a schematic diagram of a laser of the present invention withtwo shorter Nd:YVO₄ crystals that effectively act as a single crystal;illustrated is an aperture positioned between the two crystals.

DETAILED DESCRIPTION

Referring now to FIG. 1, a diode pumped laser 10 has a resonator cavity12 defined by a first high reflector mirror 14 and an output coupler 15that defines a resonator cavity. A first Nd:YVO₄ gain medium 16 ispositioned in the resonator cavity. A first diode pump source 18supplies a first pump beam 20 that is incident on a first pump face 22of first Nd:YVO₄ gain medium 16. Optionally, a back surface 30 of firstNd:YVO₄ gain medium 16 can be coated with a coating that is reflectivefor pump light from first pump beam 20.

First diode pump source 18 can be a single emitter, a broad stripeemitter or a diode bar. First diode pump source 18 can be associatedwith one or more lenses 24 in order to focus first pump beam 20 fromfirst diode source 18 so that it is incident on gain medium 16. In oneembodiment, two lenses are provided in a telescope arrangement tooptimize the size of the pump beam. First diode pump source 18 can befiber coupled. In one specific embodiment, first diode pump source 18provides a first pump beam 20 with at least 20 watts of power incidenton first pump face 22 of first Nd:YVO₄ gain medium 16.

In various embodiments, laser 10 has high efficiency with at least 35%of the pump power being converted to output power. In one embodiment,the pump power at 809 nm is converted efficiently to an output at 1064nm with a TEM₀₀ output beam. Laser 10 can be a high power laser thatproduces at least 10 watts at 1064 nm.

Referring now to FIG. 2, laser 10 includes a second diode pump source 26supplying a second pump beam 28 that is incident on a second pump face30 of first Nd:YVO₄ gain medium 16. Second diode pump source 26 can beassociated with one or more lenses 32 similar to lenses 24.

The embodiment illustrated in FIG. 2 is a Z fold geometry that includesfold mirrors 34 and 36 which can be highly reflective at 1064 nm andhighly transmissive at 809 nm.

Another embodiment of laser 10 is illustrated in FIG. 3 that includesfirst and second Nd:YVO₄ gain media with four diode pump sources. Athird diode pump source 38 produces a third pump beam 40 that isincident on first pump face 41 of second Nd:YVO₄ gain medium 43. Afourth diode pump source 42 produces a fourth pump beam 44 incident on asecond pump face 46 of second Nd:YVO₄ gain medium 43. Optionallyincluded are one or more telescopic lens arrangements 48 and 50 . In allembodiments illustrated in FIGS. 1-4, laser 10 produces an output beam52.

In various embodiments, first and second Nd:YVO₄ gain medium 16 and 43can have Nd doping levels of less than 0.5%, 0.4%, 0.3%, 0.2% and 0.1%.First and second Nd:YVO₄ gain medium 16 and 43 can have pump volumes ofat least 8 mm³, 10 mm³ or 12 mm³.

The effective absorption length of Nd:YVO₄ gain medium 16 dependsprimarily on 3 parameters: the crystal doping, the pump wavelength andthe crystal length. To increase the absorption depth beyond the typicalvalue of 4 to 8 mm, first a longer crystal can be used. A combination oflower doping and detuning of the pump wavelength away from the peak ofthe absorption can be used so that the pump power is distributedthroughout the entire crystal. This can also be achieved by broadeningthe pump bandwidth around the peak of the absorption. The “pump volume”is defined as the volume in which the gain media is excited by the pump.Typically, the pump volume is shaped like a cylinder with a diameterdetermined by the diameter of the pump light and a length determined bythe crystal length.

Typical Vanadate systems use a pump diameter of 0.7 mm and a crystallength of 4 or 8 mm. Thus the pump volume would be 1.5 or 3 mm³respectively. With a Nd doping of 1% in a 4 mm crystal or 0.5% in an 8mm crystal, approximately 90% of the pump light is absorbed. Whengreater than 13 W of pump light at the peak absorption wavelength forNd:Vanadate (809 nm) is incident on one face of the crystal, the thermallens becomes sufficiently aberrated to reduce the laser efficiency.

In one embodiment of the invention, the thermal lens is reduced byincreasing the pump mode to 1.1 mm, increasing the crystal length to 12mm and reducing the Nd doping to 0.27%. In this case the pump volume isincreased to 12 mm³. Pump powers up to 26 W on one face have been usedwithout reduced laser efficiency. For high power systems, this crystalconfiguration can be used with 26 W of pump power on each of the twofaces of the crystal.

First and second Nd:YVO₄ gain medium 16 and 43 can have lengths greaterthan 8 mm, 10 mm, 12 mm or 16 mm. The lengths of first and secondNd:YVO₄ can be achieved with a single crystal or by positioning twocrystals 16′ and 16″ (FIG. 4) in close proximity to each other or in acontacting relationship. Crystals 16′ and 16″ are positionedsufficiently close to each other to act as a single crystal. It will beappreciated that one of both of the two crystals can be coated, or anintracavity element, including but not limited to an aperture 54, can bepositioned between crystals 16′ and 16″.

Low power diode pumped solid state lasers can be “passively cooled”. Forpurposes of this disclosure passive cooling is achieved by conductingthe heat away using metal components in intimate contact with the gainmedia. These metal components may also be attached to fins or heatspreading plates. Prior to this invention, as the pump power wasincreased, “active cooling” methods become necessary to keep the laserfrom having extended warm-up times or fluctuating performance as theambient temperature was varied. Active cooling methods included: watercooled heat sinks, TE coolers and fans. With the present invention,reducing the thermal lens in a Vanadate laser permits the use of passivecooling to obtain high performance at higher pump powers and hencehigher output powers. Vanadate lasers can now be operated at 10 Waverage power or frequency doubled to 5 W average power in the greenwith only passive cooling.

In various embodiments, laser 10 is compact. Compactness is achieved bypassive cooling and/or the use of fiber-coupled diode bars as the pumpsource. Compactness is also achieved with the use of fiber-coupled diodebars. By placing the diode bars in the power supply, cooling of thediode source is eliminated from the laser head. Compactness providesgreater thermal and mechanically stability of laser 10.

First, second, third and fourth diode pump sources 18, 26, 38 and 42 canbe fiber coupled. Preferably, laser 10 is end-pumped by first, second,third and fourth diode pump sources 18, 26, 38 and 42. In oneembodiment, first, second, third and fourth diode pump sources 18, 26,38 and 42 can produce at least 20 watts of power incident upon theirrespective pump faces of first and second Nd:YVO₄ gain medium 16 and 43respectively. In the different embodiments of FIGS. 1-3, output beam 52can have a power of at least 5 watts, 7 watts and 10 watts at 532 nm.Output beam 52 can have a power of at least 10 watts, 15 watts and 20watts at 1064 nm.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Obviously, many modifications and variations will be apparentto practitioners skilled in this art. It is intended that the scope ofthe invention be defined by the following claims and their equivalents.

What is claimed is:
 1. A diode pumped laser, comprising: a first highreflector mirror and an output coupler defining a resonator cavity; afirst Nd:YVO₄ gain medium positioned in the resonator cavity, the gainmedium having a uniform doping level of less than 0.5% to reduce thermallensing and a length of at least 4 mm to absorb a substantial portion ofa pump bean; and a first diode pump source supplying a first pump beamthat is incident on a first pump face of the first Nd:YVO₄ gain medium.2. The laser of claim 1, wherein the doping level is no greater than0.4%.
 3. The laser of claim 1, wherein the doping level is no greaterthan 0.3%.
 4. The laser of claim 1, wherein the doping level is nogreater than 0.2%.
 5. The laser of claim 1, wherein the doping level isno greater than 0.1%.
 6. The laser of claim 1, wherein the doping levelis 0.27%.
 7. The laser of claim 1, wherein a length of the first Nd:YVO₄gain medium is at least 8 mm.
 8. The laser of claim 1, wherein the firstNd:YVO₄ gain medium is end-pumped by the first pump beam.
 9. The laserof claim 1, wherein the first Nd:YVO₄ gain medium has a pump volume ofat least 8 mm³.
 10. The laser of claim 1, wherein the first Nd:YVO₄ gainmedium has a pump volume of at least 10 mm³.
 11. The laser of claim 1,wherein the first diode pump source is a fiber coupled diode pumpsource.
 12. The laser of claim 11, wherein the first diode pump sourceprovides a pump beam with at least 20 watts of power incident on thefirst pump face of the first Nd:YVO₄ gain medium.
 13. The laser of claim1, further comprising: a second diode pump source supplying a secondpump beam that is incident on a second pump face of the first Nd:YVO₄gain medium.
 14. The laser of claim 13, wherein the first and seconddiode pump sources are fiber coupled.
 15. The laser of claim 14, whereinthe second diode pump source provides a pump beam with at least 20 wattsof power incident on the second pump face of the first Nd:YVO₄ gainmedium.
 16. The laser of claim 13, further comprising: a second Nd:YVO₄gain medium positioned in the resonator cavity; a third diode pumpsource supplying a third pump beam that is incident on a first pump faceof the second Nd:YVO₄ gain medium; and a fourth diode pump sourcesupplying a fourth pump beam that is incident on a second pump face ofthe second Nd:YVO₄ gain medium.
 17. The laser of claim 16, wherein thefirst, second, third and fourth diode pump sources are fiber coupleddiode pump sources.
 18. The laser of claim 1, wherein the laser producesan output beam with a power of at least 10 watts at 532 nm.
 19. Thelaser of claim 1, wherein the laser produces an output beam with a powerof at least 20 watts at 1064 nm.
 20. The laser of claim 1, wherein thelength of the first Nd:YVO₄ gain medium is greater than 10 mm.
 21. Thelaser of claim 1, wherein the length of the first Nd:YVO₄ gain medium isgreater than 12 mm.
 22. The laser of claim 1, wherein the length of thefirst Nd:YVO₄ gain medium is greater than 16 mm.
 23. A diode pumpedlaser, comprising: a first high reflector mirror and an output couplerdefining a resonator cavity; a first Nd:YVO₄ gain medium positioned inthe resonator cavity with uniform doping of 0.5% or less and a pumpvolume of at least 8 mm³ to reduce thermal lensing; and a first diodepump source supplying a first pump beam that end pumps a first pump faceof the first Nd:YVO₄ gain medium.
 24. The laser of claim 23, wherein thepump volume is at least 10 mm³.
 25. The laser of claim 23, wherein thepump volume is at least 12 mm³.
 26. The laser of claim 23, wherein adoping level of the first Nd:YVO₄ gain medium is 0.27%.
 27. The laser ofclaim 23, wherein a length of the first Nd:YVO₄ gain medium is greaterthan 8 mm.
 28. The laser of claim 23, wherein the first diode pumpsource is fiber coupled.
 29. The laser of claim 28, wherein the firstdiode pump source provides a pump beam with at least 20 watts of powerincident on the first pump face of the first Nd:YVO₄ gain medium. 30.The laser of claim 23, further comprising: a second diode pump sourcesupplying a second pump beam that is incident on a second pump face ofthe first Nd:YVO₄ gain medium.
 31. The laser of claim 30, wherein thefirst and second diode pump sources are fiber coupled.
 32. The laser ofclaim 31, wherein the second diode pump source provides a pump beam withat least 20 watts of power incident on the second pump face of the firstNd:YVO₄ gain medium.
 33. The laser of claim 30, further comprising: asecond Nd:YVO₄ gain medium positioned in the resonator cavity; a thirddiode pump source supplying a third pump beam that is incident on afirst pump face of the second Nd:YVO₄ gain medium; and a fourth diodepump source supplying a fourth pump beam that is incident on a secondpump face of the second Nd:YVO₄ gain medium.
 34. The laser of claim 33,wherein the first, second, third and fourth diode pump sources are fibercoupled.
 35. The laser of claim 23, wherein the laser produces an outputbeam with a power of at least 10 watts at 532 nm.
 36. The laser of claim23, wherein the laser produces an output beam with a power of at least20 watts at 1064 nm.
 37. A diode pumped laser, comprising: a first highreflector mirror and an output coupler defining a resonator cavity; afirst Nd:YVO₄ gain medium positioned in the resonator cavity and havinga uniform doping level of less than 0.5% to reduce thermal lensing and alength greater than 8 mm to absorb a substantial portion of a pump beam;and a first diode pump source supplying a first pump beam that isincident on a first pump face of the first Nd:YVO₄ gain medium.